executor: move current-thread into crate (#1447)

The `CurrentThread` executor is exposed using a feature flag.

Refs: #1264

ui-tests/tests/features.rs

@@ -0,0 +1,9 @@
+#[test]
+fn features() {
+    let t = trybuild::TestCases::new();
+
+    #[cfg(feature = "executor-without-current-thread")]
+    t.compile_fail("tests/ui/executor_without_current_thread.rs");
+
+    drop(t);
+}